Tag Archives: Perl

Perl to JavaScript Data via JSON

I have a Perl script monitoring serial ports for sensor data, that data is then to be accessed in a web page via AJAX so needs to be readable by JavaScript. I’m using JSON as the data format to transport the data.

Writing JSON in Perl

Install the JSON module with CPAN if not already installed. Then simply construct a hash table, or a hash of hashes to create a name hierarchy. Once the hash is created, use the to_json function to format the hash as a JSON string and write that string to a text file.

1
2
3
4
5
6
7
8
my $json->{"temp"} = {"f" => ($ad1 * 100.0 / 0x3ff) * 9 / 5 + 32, "c" => ($ad1 * 100.0 / 0x3ff)};
$json->{"water"} = $dio4;
 
$json_text = to_json $json;
 
open FILE, ">xbee.json";
print FILE $json_text;
close FILE;

Reading JSON in JavaScript

Now that JSON file can be accessed from a web browser to display the results. Using jQuery there is a .getJSON function that will read the file and unpack the JSON format into a JavaScript variable. Once in the variable, as data is here, the hashes written in Perl are accessed as child objects of the variable. Eg. the hash water is simple accessed as data.water in Javascript.

1
2
3
4
5
6
7
8
$.getJSON('xbee.php', function(data) {
 $('#basement_temp').html(data.temp.f.toFixed(1));
 if (data.water != '0') {
$('#basement_water').removeClass('sensor_alert').addClass('sensor_ok');
 } else {
$('#basement_water').removeClass('sensor_ok').addClass('sensor_alert');
 }
 });

Twitter Water Sensor Software

Start with the modules programmed, wired and plugged in as described in the last 2 posts. The modules should be communicating and sending the sensor samples. We now need to read the data from the serial port and write a tweet when the sensor indicates a low on DIO4. Before writing and running this code, you will need to have set up a Twitter account and set up an Application. Then follow this account from your main twitter account, enabling mobile messaging if you want to receive a text message when an event occurs.

This is developed on Ubuntu, but using the Perl scripting language that should be portable to other platforms. Use CPAN to install the dependent modules:

  • Device::SerialPort for communications with the XBee
  • Tie::CharArray for splitting a string into a character array
  • Net::Twitter  provides a class to interface with Twitter

Open and read data from the serial port. We convert the received binary string into a character array of the ordinal values for easier processing. Then copy characters read into a processing buffer so we can handle multiple reports coming in a single read or partial reads.

1
2
3
4
5
6
7
8
9
my $port=Device::SerialPort->new("/dev/ttyS0");</pre>
while ($timeout>0)
{
    my ($count,$saw)=$port->read(255); # will read _up to_ 255 chars
    tie my @chars, 'Tie::CharArray::Ord', $saw;
    for ($i = 0; $i < $count; $i++) {
        push(@buffer, @chars[$i]);
        printf("0x%02x ", @buffer[$i]);
    }

We then run a series of checks on the header information to verify the packet is received correctly. First check the header character for proper framing, or throw away characters until we find the next frame header.

1
2
3
4
5
6
# check frame header
if (@buffer[0] != 0x7e) {
    printf("Bad Frame\n");
    shift(@buffer);
    next;
}

Then verify all the characters expected have been received by checking the packet size.

1
2
3
4
# check all chars are now in buffer
$size = @buffer[2] + @buffer[1] * 0x100;
printf("Size:     %d\n", $size);
if ($size > scalar(@buffer) - 4) { next; }

Then verify the contents of the package by running the checksum operation.

1
2
3
4
5
6
7
8
9
10
11
12
# check checksum
$sum = 0;
for ($i = 0; $i < $size ; $i++) {
    $sum += @buffer[$i + 3];
}
$sum = 0xff - $sum % 0x100;
if ($sum != @buffer[$size + 3]) {
    printf("Checksum: FAIL %02x != %02x\n", $sum, @buffer[$size + 3]);
    next;
} else {
    printf("Checksum: PASS %02x\n", $sum);
}

If contents are complete and intact, copy message to a packet buffer.

1
2
3
4
my @packet;
for ($i = 0; $i < $size + 4 ; $i++) {
    push(@packet, @buffer[$i]);
}

If an packet received is an IO sample packet, process the IO message. Then remove this packet from the input buffer.

1
2
3
4
if (@buffer[3] == 0x92) {
    ProcessIO(\@packet);
}
for ($i = 0; $i < $size + 4; $i++) {shift(@buffer);}

In the ProcessIO sub routine, extract the packet information. Fetch the device ID of the sending XBee, the 16 bit ID, packet type, and read digital IO 4.

1
2
3
4
5
6
7
8
9
10
11
12
sub ProcessIO {
    my @packet = @{(shift)};
     
    my $xbeeid = sprintf("%02x%02x%02x%02x%02x%02x%02x%02x", @packet[4], @packet[5], @packet[6],
        @packet[7], @packet[8], @packet[9], @packet[10], @packet[11]);
    printf("DeviceID: %s\n", $xbeeid);
 
    printf("Net MyID: %02x%02x\n", @packet[12], @packet[13]);
     
    printf("PackType: %02x\n", @packet[14]);
     
    my $dio4 = (@packet[20] & 0x10) != 0;

Once we have this information, we compare the device ID to verify it is the sensor device we are interested in. Then check if the DIO4 line has been brought low from previously being high since we only want to tweet the first time water is detected, not every second. This code is taken from the sample code in the Net::Twitter documentation. Net::Twitter->new creates and authenticates a connection with Twitter, then ->update posts a tweet. A time stamp is added to the message, otherwise it may be rejected as a duplicate tweet.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
if ($xbeeid eq $xbee_basement)  {       if (($dio4 == 0) && ($last_water == 1))         {           printf("DETECTED WATER!!\n");           my $nt = Net::Twitter->new(
            traits   => [qw/OAuth API::REST WrapError/],
            consumer_key        => $consumer_key,
            consumer_secret     => $consumer_secret,
            access_token        => $token,
            access_token_secret => $token_secret,
        );
 
        $now_string = strftime "%H:%M", localtime;
        $tweet = sprintf("At %s Basement sensor 4 detected water", $now_string);
 
        my $result = $nt->update($tweet);
        #print Dumper $result;
    }
    $last_water = $dio4;

Then just run the script from a terminal.
Perl xbee.pl

Parsing Misterhouse Serial Data

The sensors transmit data in the form “Address,Port,Value” A typical input would be “A1,T1,30.0” for module 1, temperature sensor 1, and 30 deg C.

In this case only temp data is read by this. split is used to segment the three elements, the temperature is converted to Fahrenheit, and the result stored in a hash.

1
2
3
4
5
6
7
$xbee_serial = new Serial_Item(undef, undef, 'serial1');
if (my $data = said $xbee_serial)
{
    my @values = split(',', $data);
    my $temp = $values[2] * 9 / 5 + 32;
    $xbee_data{ $values[0].$values[1] } = $temp;
}